The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
1. Field of the Invention
This invention pertains to a method and apparatus of thermal, non-destructive inspection of the surface of a material.
2. Background of the Invention
Thermal, non-destructive inspection systems are an inspection tool for both manufacture and in-service applications. These inspection systems are based on the application of heat onto the surface of the structure to be inspected. Typical heat sources used are flash lamps. The short duration intense light is absorbed by the surface to be inspected and a temperature rise above ambient temperature is produced. A device such as in an infrared detector is used to measure small differences in the surface temperature as the sample cools down. The temperature time history is recorded and stored for analysis using theoretical models. Model based thermal inspections are possible when extraneous factors are minimized so that the theoretical models best fit the data.
Thermal inspection of a sample material can be achieved by single-sided inspection or by through-transmission inspection. Single-sided inspection means that the flash lamps that heat the sample under test are on the same side of the sample as the infrared detector that thermally inspects the temperature of various points on the sample versus time as the sample cools down after being heated. Through-transmission inspection requires that the infrared detector be on a side of the sample under test opposite to the flash heat lamps used to heat the sample. Through-transmission measurements can utilize either one or more flash heat lamps for thermal inspection, depending on the size of the sample.
Measurements gleaned from thermal inspection generally include locating defects, including disbond defects in a sample under test, integrity of a weld on an aircraft, the detection of corrosion and the detection of paint thicknesses. The detection of corrosion is found by determining the volumetric heat capacity on all points of a sample under test (specimen) gleaned from data received by an infrared detector that is processed based on a theoretical model. The location of defects is determined by examining a thermal diffusivity image obtained by processing the specimen""s thermal responses. The thermal diffusivity is defined as the ratio between the thermal conductivity of the sample under test divided by the volumetric heat capacity of the sample under test. Generally, these measurements are made by first rapidly heating the sample under test by one or more flash lamps, then measuring the rate of cool down over time on different points of the sample under test by an infrared detector.
The invention is a thermal, non-destructive evaluation system and method comprising a synchronized electronic shutter system (SESS) where each flash heat lamp and a detector for measuring infrared thermal energy such as an infrared camera all contain shutters. The opening and closing of each shutter is electronically controlled so that the data read when the shutter for the infrared detector is opened is both accurate and precise, whether it be for single-sided thermal inspection or for through-transmission inspection. Upon receiving a signal from a source like a computer to conduct a thermal inspection of an object under test, the shutter to the infrared detector is opened while the shutter(s) to the flash heat lamp(s) is/are closed, enabling the infrared detector to acquire background images for the object under test. Following this activity, the shutter to the detector is closed at the same time the shutter(s) to the flash heat lamp(s) is/are opened. While the shutter(s) to the flash heat lamp(s) is/are open and the shutter to the infrared detector is closed, the flash heat lamp(s) are fired or actuated (flash duration is typically 0.008 seconds) to heat the object under test. Because the shutter to the infrared detector is closed, photons reflected off the object under test do not influence the infrared detector. Also, because the shutter(s) of the flash heat lamp(s) is/are open, the/these shutter(s) is/are not heated. After actuation of the flash heat lamp(s), the shutter(s) on the flash heat lamp(s) is/are closed to cover the flash heat lamp(s) at the same time the shutter to the infrared detector is opened to start thermal data acquisition over a period of time while the object under test cools down. Because the shutter(s) to the flash heat lamp(s) now cover the flash heat lamps, residual transient effects originating from a recently fired flash heat lamp(s) do not reach the infrared detector and thus do not effect the data being acquired. Also, because the shutter(s) for the flash heat lamp(s), which may be made from a material having high thermal conductivity, was/were not covering the flash heat lamp(s) during actuation, the shutter(s) over the flash heat lamp(s) are cool, thus eliminating the erroneous effects of infrared radiation emanating from a material that covers the flash heat lamp(s) and being reflected off the object under test and back into the infrared detector. The method of this invention uses an SESS that operates the detector shutter and the flash heat lamp shutter(s) so as to complement each other during times of opening and closing.